add(layout/padding): layout configuration for padding

With minor code layout fixes

examples/container.zig

@@ -26,14 +26,14 @@ pub fn main() !void {
     var container = try App.Container.init(allocator, .{
         .border = .{
             .color = .blue,
-            .sides = .{
-                .left = false,
-                .bottom = false,
-            },
             .corners = .rounded,
             .separator = .{ .enabled = false },
         },
-        .layout = .{ .gap = 3, .direction = .horizontal },
+        .layout = .{
+            .gap = 3,
+            .padding = .all(2),
+            .direction = .horizontal,
+        },
     });
     try container.append(try App.Container.init(allocator, .{
         .border = .{ .color = .light_blue, .corners = .squared },
@@ -42,6 +42,10 @@ pub fn main() !void {
     try container.append(try App.Container.init(allocator, .{
         .border = .{ .color = .light_blue, .corners = .squared },
     }));
+    try container.append(try App.Container.init(allocator, .{
+        .border = .{ .color = .light_blue, .corners = .squared },
+        .rectangle = .{ .fill = .blue },
+    }));
     defer container.deinit();
 
     // NOTE: should the min-size here be required?